There is a demand to increase the flying speed of a helicopter. Ina forward flight of a helicopter, by cyclic control for changing pitch angles of the blades of the rotor, a rotation plane of the rotor is forward-tilted to generate thrust in the forward direction. Referring to FIG. 1, as for a rotor of a conventional helicopter, an airspeed of a forward blade 101 is represented by the sum of a rotation speed (a circumferential speed) and a forward speed of the helicopter, and an airspeed of a backward blade 102 is represented by the difference between the rotation speed and the forward speed of the helicopter. When the forward speed of the helicopter is high, a shock wave is generated at the forward blade 101 of high airspeed, and a stall occurs at the backward blade 102 of low airspeed. When the stall occurs at the backward blade 102, a rotation plane of the rotor is backward-tilted and accordingly a thrust in a forward direction cannot be generated. For the above-mentioned reason, the limit of forward speed of a single rotor helicopter has been considered to be about 200 knots (370 km/hr).
Japanese patent publication (JP-A-Heisei 9-48398) discloses a lead-lag constraint device. The lead-lag constraint device includes a stopper for limiting displacement of a blade in a lead-lag direction. The stopper moves to a position of almost entirely limiting the displacement of the blade in the lead-lag direction in ground rotor run and moves to a position of allowing the displacement of the blade in air rotor run.